This invention relates to radar systems, and more particularly to pulsed radar systems used for surveillance.
Radar systems are in widespread use for military, commercial, and private purposes. Radar systems have well-known characteristics, in that long-range detection of small targets is known to require transmission of more power, higher-gain antennas, and/or more sensitive receivers than that or those required for short-range detection of large targets. Among the characteristics of radar systems used for detecting targets at long range are those relating to range ambiguity, which has to do with reception of signals returned from a target lying beyond the range defined by the pulse repetition interval, which may make the distant target appear to be near the radar system. Another such characteristic of radar is that of range eclipsing, which has to do with the inability of a radar receiver to receive return signals during the pulse transmission interval.
A conventional solution to range eclipsing is to vary the pulse repetition interval, so that the transmitted pulses are staggered over time, thereby allowing the receiver to periodically xe2x80x9cseexe2x80x9d returned signals at times which would otherwise be lost or eclipsed. The eclipsing still occurs for each individual pulse train, but the totality of the radar returns over time includes information which fills in the gaps attributable to the individual transmitted pulse trains. The tradeoff is that a longer time is required to produce all the information required for an uneclipsed view of the region. Another possible solution to range eclipsing is to reduce the duty cycle of the radar by reducing the transmitted pulse duration, to thereby reduce the duration of the eclipsing. The reduction of the pulse duration, however, tends to reduce the transmitted energy, which reduces the range sensitivity, which again requires a longer period of integration in order to obtain the same effective range.
Another possible solution to range eclipsing is to reduce the duty cycle of the radar by increasing the pulse repetition interval to thereby move the increased range interval to a distant range not of interest. The reduction of the duty cycle and increase in the pulse repetition interval, however, tends to consume additional radar resources resulting in a greater overall time required for completion of a surveillance scan.
Conventional range ambiguity resolution techniques require transmission of additional signals with additional dwells for resolving the range interval of the ambiguous target. The additional dwells or transmissions consume additional radar resources, resulting in a greater overall time required for completion of a surveillance scan.
Improved radar systems that provide unambiguous and/or uneclipsed range coverage are desired.
A method for transmitting and receiving radar signals according to an aspect of the invention includes transmitting time-sequential pulses including first and second pulses at first and second mutually different frequencies, where the first and second pulses being separated by a first time duration, and receiving radar return signals at the first frequency during times corresponding to the first time duration and during a second time duration that begins after transmission of the second pulse, which second time duration is different from the first time duration, and also receiving the second frequency during a third time duration. The method according to this aspect of the invention also includes processing the radar return signals to provide signals representative of targets lying in an uneclipsed range nominally corresponding to the sums of (a) the sum of the first and second time durations and (b) the duration of the second pulse. In one mode of the method according to this aspect of the invention, the second time duration is less than the first time duration. In another mode of the method according to this aspect of the invention, the third time duration follows the first and second time durations.
According to a further aspect of a method according to an aspect of the invention, the additional step of transmitting additional pulses at additional frequencies is performed, where the additional frequencies are different from each other and from the first and second frequencies. These additional pulses are transmitted following the second pulse, and at least some of the additional pulses are transmitted later than the immediately preceding pulse by alternating ones of the first and second time durations. In this further aspect of a method according to an aspect of the invention, the further step is performed of receiving radar return signals at a frequency corresponding to one of the additional frequencies during a time interval which begins at a time following that pulse on which the one of the additional frequencies was transmitted by the sum of (a) one of the first and second time durations and (b) a pulse duration.
In yet another mode of a method according to an aspect of the invention, the further step is performed of processing the radar return signals at the additional frequencies together with at least some of the radar return signals at the first and second frequencies to produce the signals representative of targets. Another mode of the method according to an aspect of the invenition further comprises, with a particular antenna pointing, repeating a particular number of times the steps of transmitting time-sequential pulses, receiving radar return signals, transmitting additional pulses at additional frequencies, receiving radar return signals at the additional frequencies, and processing the radar return signals.
Another method according to an aspect of the invention is for transmitting and receiving radar signals. This method comprises, with the radar antenna pointed in a particular direction, of generating a timing signal train including at least one sub-timing signal train, where each of the sub-timing signal trains includes at least first, second and third alternating and sequential pulse repetition intervals characterized by one of a xcex2:1 and a 1:xcex2 ratio of (a) the duration from the start time of a given first pulse repetition interval to the start time of a second pulse repetition interval, where the second pulse repetition interval next follows the first pulse repetition interval, divided by (b) the duration from the start time of the second pulse repetition interval to the start time of a third pulse repetition interval next following the second pulse repetition interval, where xcex2 is given by 1+(xcex1D), where xcex1 is equal to or greater than unity, and D is the duty cycle of the radar. In this method, in response to each sub-timing signal train, electromagnetic radar pulses are transmitted from the antenna such that the frequency of transmission in response to the first pulse of the sub-timing signal train is at a first radio frequency and the frequency of transmission in response to the second pulse of the sub-timing signal train is at a second radio frequency, different from the first radio frequency by an amount which allows subsequent separation of signals at the first and second frequencies. In addition, this method includes the reception, during that interpulse period immediately following the first pulse, of electromagnetic radar return signals originating in response to the first pulse, to thereby generate first received signals, reception, during that interpulse period immediately following the second pulse, of electromagnetic radar return signals originating in response to the first pulse, to thereby generate second received signals, and reception, during the third pulse repetition interval, of electromagnetic radar return signals originating in response to the second pulse, to thereby generate third received signals. The first and second received signals are concatenated, with a delay therebetween no less than the duration of the second pulse, to thereby produce a concatenated return signal originating from the first radio frequency pulse, which includes information relating to the presence or absence of targets in an unambiguous range interval extending from the end of the first pulse to near the beginning of the second pulse, and extending from the end of the second pulse to near the beginning of the third pulse repetition interval, but does not include information relating to the presence or absence of targets attributable to signals received during transmission of the second pulse. The third received signals are delayed by a time duration equal to the second pulse repetition interval, to thereby produce a delayed third return signal including information relating to the presence or absence of targets in the interval which coincides with ranges extending at least from the beginning of the second pulse to near the beginning of the third pulse repetition interval relative to the start of the sub-pulse train. Finally, this method combines the information from the concatenated return signal with the information from the delayed return signal to thereby produce signals representing the presence or absence of targets in the interval extending from the end of the first pulse to near the beginning of the third pulse repetition interval range unambiguously and range uneclipsed. The combining may include integration, which may be coherent or nonncoherent. In a particular mode of this method, the sub-timing-signal train includes N+1 pulse repetition intervals, where N is greater than or equal to two, N pulses of electromagnetic radiation are transmitted during the first N of the N+1 pulse repetition intervals, each at a different RF frequency, and the combining includes the step of noncoherent integration. In another particular mode of this method, the sub-timing-signal train includes N+1 pulse repetition intervals where N is greater than or equal to two, N pulses of electromagnetic radiation are transmitted during the first N of the N+1 pulse repetition intervals, wherein each pulse following the first two pulses represents transmission of electromagnetic energy at a frequency equal to that of the pulse transmitted two pulse repetition intervals earlier, and the combining includes at least one of the steps of (a) coherent and (b) noncoherent integration. In yet another particular mode of this method, the timing signal train includes M of the sub-timing signal trains, where M is greater than or equal to one, the number of pulse repetition intervals in each of the sub-timing signal trains need not be equal, and the combining includes at least one of the steps of (a) coherent integration and (b) non-coherent integration. According to a variant mode of a method according to an aspect of the invention, the further steps are included of re-steering the antenna following the generation of the timing signal train, and generating a second timing signal train that need not have the same number of the sub-timing signal trains nor the same number of the pulse repetition intervals per the sub-pulse train as earlier the timing signal train nor the same method of the combining. In one manifestation of a mode of a method according to an aspect of the invention, the timing signal train includes M of the sub-timing signal trains, where M is greater than or equal to one, the number of pulse repetition intervals in each of the sub-timing signal trains need not be equal, and the combining includes the step of coherent and/or non-coherent integration. In this manifestation, the further steps may be included of re-steering the antenna following the generation of the timing signal train and generating a second timing signal train that need not have the same number of the sub-timing signal trains nor the same number of the pulse repetition intervals per the sub-pulse train nor the same method of the combining as the earlier timing signal train.